The present invention relates to surface-modified "plate-like", e.g., platelet-shaped, pigments having improved dispersibility and to their preparation and use.
The handling of paints, printing inks, and the like containing platelet-shaped pigments is problematic inasmuch as the pigments easily settle, due to their size and density compared with the paint medium, and can then agglomerate give a very solid sediment cake. In practice, this usually cannot be prevented, unless the printing inks are kept in constant agitation, for example, by stirring or recirculation.
Thus, inter alia, many methods have been developed in order to solve the problem of handling platelet-shaped pigments in paints and printing inks.
Redispersion can be facilitated by adding to the coating compositions, such as paints, varnishes, printing inks, and the like, additives which either result in selective flocculation (a "house-of-cards" effect), structurally viscous and/or thixotropic behavior, steric repulsion and/or electrostatic repulsion of the pigments.
The addition of thickeners (thixotropic reagents), which in the usual cases prevent sedimentation by increasing the viscosity of the paint medium (EP-0,198,519; DE-OS 1,805,693), can be used in the case of printing inks only to a limited extent, since, if the ink is to be printed, a certain narrow viscosity range must be maintained.
The production of structural viscosity on the pigment surface by the addition of finely divided silica gels or bentonites is also only of limited use, since the gloss of the pigment coating may suffer; and, on the other hand, structural viscosity in printing inks is usually undesirable.
Furthermore, all these additives can have an adverse effect on the quality of the coating. In particular the brilliance of nacreous pigments and the uniformity of the coating may be impaired. In general, the higher the concentration of the additives used, the greater the impairment.